John Gruber, under "miscellaneous" in his review of HomePod:
People seem to naturally think they need to pause between saying “Hey Siri” and issuing the command or query, but in my experience you don’t need to. In this review, I’ve been punctuating directives with a comma after “Hey Siri”, but verbally you can speak without any pause: “Hey Siri what’s the temperature”. This is true not just for HomePod but any other device you own with “Hey Siri” enabled.
I'm one of those people. Me, in my review of the Google Pixel 2 XL and Android 8.1:
I can say “OK Google” at any time and, again instantly make my request. And I do mean instant, unlike Siri, there is no pause needed between “OK Google” and my request. With Siri I need to wait for the “ding” sound.
Somehow John's experience and my experience differ. On all of the iPhones I've owned that were Siri-enabled I have needed to pause. And the pause was doubly long when connected to something, like my car or a speaker, using Bluetooth or AirPlay. Perhaps a very recent software update has removed this pause or I simply wasn't doing something correctly.